The small papovavirus SV40 induces tumors in experimental animals, and transforms cells in culture. Preliminary experiments have shown that only the early region of the SV40 genome contributes to transformation. Both gene products of the SV40 early region (small-t and Large-T proteins) as well as the viral origin of early transcription, may be required. Viral mutants defective in large-T (TsA mutants) or in small-t (dl 54/59 mutants) exist. Using rat and hamster embryo fibroblasts transformed by wild type SV40 and by these mutants we will attempt to link the various changes associated with the transformed state (e.g. in growth properties, in synthesis of plasminogen activator, in metabolic state and in the ability to form tumors in nude mice) with the presence of functional small-t or Large-T antigen. We will select from rat embryo fibroblast transformants revertants which have lost specific aspects of the transformed phenotype and attempt to correlate this reversion with specific alterations in the integrated viral genome. We now know that certain of the dl 54/59 mutants (e.g. 2007) may be partially leaky for some small-t gene function. We will characterize cells transformed by these mutants and compare the phenotype to cells transformed by WT virus and by the tight mutants. We will also proceed with the isolation of TsA/tight dl 54/59 double mutants and to characterize cells transformed by these viruses. We have found that the dl 54/59 viruses are tumorigenic but that the effect of the presence of small-t is to increase the incidence and decrease the latency of tumors. We will carry out experiments to test if small-t may be acting by a mechanism similar to those proposed for tumor promoters such as PMA. Finally we will proceed with the isolation of control mutants of SV40 lacking sequences important for early message transcription or processing.